This proposal describes a research program in bioanalytical chemistry and nanostructured materials aimed at developing a new class of metal nanoparticles for ultrasensitive nonisotopi detection of biological molecules. Recent research in our group has identified a novel type of silver colloidal nanoparticles that are able to enhance the efficiencies of Raman scattering by as much as 10exp14 to 10exp15 folds. This enormous enhancement allows detection and spectroscopy of single molecules at room temperature. The proposed research will capitalize upon this discovery, and will systematically screen, characterize and enrich these novel nanoparticles. This work should yield a new detection technology with broad biomedical applications, including DNA sequencing and HIV detection. A key concept is to rapidly screen a heterogeneous population of silver and gold colloidal nanoparticles for novel optical properties. The size, shape, an optical properties of selected particles will be measured by an integrated optical and atomic force microscope. By probing one particle at a time, this work will overcome the problem of sample inhomogeneity caused by variations in particle size, shape, and surface defects. Systematic studies will examine the factors responsible for efficient optical enhancement as well as the characteristics and origins of intermittent photon emission in single nanoparticles, Methodologies will also be developed for enriching these particles and for encapsulating them in polymer films. The enriched nanoparticles will be used to detect and identify single native biomolecules such as nucleotide bases and amino acids. This detection scheme could solve a long-standing problem in single-molecule DNA sequencing, in whic a single DNA fragment is cleaved by an exonuclease enzyme and the cleaved nucleotides are detected and identified one by one. Surface-enhanced Raman DNA probes will be developed by conjugating oligonucleotides and spectroscopic tag to metal nanoparticles. A large number of distinct Raman tags are available fo encoding oligonucleotide probes, which could be used to monitor the expression of many genes in normal and diseased cells. A potential clinical application i to determine the viral RNA load in infectious diseases such as HIV and hepatitis.